Method for producing sintered cobalt-base alloy containing gold

ABSTRACT

COBALT-BASE ALLOYS CONTAINING GOLD ARE PREPARED BY A POWDER METALLURGY PROCESS COMPRISING DISSOLVING A PREDETERMINED AMOUNT OF GOLD IN SUFFICIENT AQUA REGIA TO FORM A SOLUTION, ADDING A SUFFICIENT AMOUNT OF A COBALT CONTAINING CONSTITUENT TO THE SOLUTION TO ACHIEVE THE COBALT TO GOLD RATIO IN THE RESULTING MIXTURE THAT IS DESIRED IN THE ALLOY, DRYING THE MIXTURE TO FORM A POWDER, HEATING THE POWDER IN A REDUCING ATMOSPHERE UNDER CONTROLLED TIME AND TEMPERATURE CONDITIONS, COMPACTING THE RESULTING REDUCED POWDER INTO A SHAPED ARTICLE AND SINTERING THE SHAPED ARTICLE IN A REDUCING ATMOSPHERE UNDER CONTROLLED TIME AND TEMPERATURE CONDITIONS.

United States Patent O Int. Cl. 1322f N00 US. Cl. 75211 6 (Haims ABSTRACT OF THE DISCLOSURE Cobalt-base alloys containing gold are prepared by a powder metallurgy process comprising dissolving a predetermined amount of gold in sufiicient aqua regia to form a solution, adding a sufiicient amount of a cobalt containing constituent to the solution to achieve the cobalt to gold ratio in the resulting mixture that is desired in the alloy, drying the mixture to form a powder, heating the powder in a reducing atmosphere under controlled time and temperature conditions, compacting the resulting reduced powder into a shaped article and sintering the shaped article in a reducing atmosphere under controlled time andtemperature conditions.

BACKGROUND OF THE INVENTION This invention relates to cobalt-base alloy containing gold. More particularly it relates to powder metallurgy process for producing cobalt-base alloys containing gold.

Generally, cobalt-base alloys containing gold have been prepared by one of two methods. In one method the cobalt and gold are heated to form a melt. Although uniformity of the gold and cobalt are achieved while in the molten state, the melt is cast, and during solidification of the melt, the gold tends to migrate to the grain boundaries of the resulting alloy. Since the gold migrates to the grain boundaries there is a non-uniformity of the gold that is detrimental to the performance of the alloy. To overcome this detriment, the alloys, after casting, were subjected to heat treatments in excess of about 100 hours.

In the second method, cobalt and gold powders, along with other powdered metals that are desired in the alloy, are blended together to form an admixture which is thereafter compacted and sintered. The second method depends upon complete dilfusion of the metals to form a uniform alloy. In practice, a uniform alloy is not, generally, achieved in every instance.

It is believed, therefore, that a process that provides a cobalt-base alloy having gold as a component thereof that is relatively uniformly dispersed throughout the alloy, is an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this invention to provide a process for producing cobalt-base alloys containing gold.

It is an additional object of this invention to improve the uniformity of dispersion of gold in cobalt-base alloys.

It is a further object of this invention to provide a process which is particularly suited to producing cobaltiron-gold alloys.

The foregoing objects are achieved in one aspect of this invention by dissolving a predetermined amount of gold in sufiicient aqua regia to form a solution, mixing a sufiicient amount of a cobalt-containing constituent with the solution to achieve the weight ratio of cobalt to gold that is desired in the alloy, drying the resulting mixture to form a powder, heating the powder in a reducing atmosphere from about 500 C. to about 1000 C. for greater than about one hour to achieve conversion of the metals therein to the metallic state, compacting the resulting partially alloyed powder into a shaped article and sintering the shaped article for at least about one hour at a temperature of from about 1000 C. to about 1250 C. to density the article and complete the formation of the alloy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above description of some of the aspects of the invention.

Although any amount of aqua regia can be used that will dissolve the gold, concentrations of gold of from about 3.85% to about 4.15% are preferred. The aqua regia-gold solution is generally formed by adding the gold in the form desired to the aqua regia containing one part concentrated nitric acid and 3 parts concentrated hydrochloric acid. Since the gold-aqua regia solution can be stored for periods of time prior to use, saturated solutions are generally not desirable since ambient temperature changes-can cause precipitation of gold from the solution.

The desired amount of cobalt containing constituent and the gold-aqua regia solution are mixed together to achieve a cobalt to gold weight ratio of from about 1:1 to about :1. If large weight ratios of cobalt to gold are desired, such as from 15:1 to 100:1, then Weaker gold concentrations within the foregoing range are desired to insure that the cobalt and gold are uniformly dispersed in the resulting mixture.

The cobalt-containing constituent can be a slurry, solution or powder metal. Additionally the cobalt-containing constituent can have nickel or iron in minor amounts, that is, less than 50% by weight of cobalt. Additionally, particularly if the cobalt-containing constituent is in powder form, water is added to achieve the desired consistency that enables conventional agitation and pumping means to be used.

After the mixture of the cobalt constituent and goldaqua regia solution is formed, the mixture is dried by a suitable means, such as a steam kettle, to form a relatively free-flowing powder.

The resulting powder is heated in a reducing atmosphere, preferably hydrogen at from about 500 C. to about 1000 C. for at least about one hour. Generally, four hours is the longest time required to reduce or deoxide the powder to insure that all metallic constituents are in the metallic state and to form a partially alloyed powder. By partially alloyed, it is meant that at least 10% by weight of the gold is dissolved in the cobalt. The average particle size of the partially alloyed powder is from about 3 to about 10 micrometers with a maximum particle size of about 20 micrometers.

The partially alloyed powder is subjected to sufficient pressure to form a shaped article, that is, an article that will retain its shape without external support.

The shaped article is, thereafter, sintered by heating to a temperature of above 1000 C. to about 1250 C. in a reducing atmosphere, preferably a hydrogen atmosphere. The time of sintering will depend upon the temperature. In general the lower the temperature the longer the time required to achieve the desired density. Generally at least 8 hours is necessary at 1250" C. and at 1000 C. less than 48 hours is sufiicient to achieve a density of 93% of theoretical. t

In order to more fully explain the foregoing invention, the following detail examples are presented. All parts, proportions and percentages are by weight unless otherwise indicated.

EXAMPLE 1 About 4 parts of gold are added to 32 parts by weight of aqua regia. An admixture of 84 parts of cobalt powder and 12 parts of iron powder is prepared and added to the gold-aqua regia solution after the gold has dissolved. Sulficient water is added to achieve good agitation in a steam kettle. After drying for about 8 to 16 hours depending upon the temperature of the steam and agitation, a powder is formed. The dried powder is placed in a furnace having a hydrogen atmosphere. The dried material is heated at about 900 C. for about 3 hours. After screening through a 100 mesh screen, the powder is compacted to a density of about 55% of theoretical to form a shaped article. The article is sintered in a furnace having a hydrogen atmosphere at a temperature of about 1250 C. for about 16 hours. During the sintering, a liquid phase is formed which is believed to be a cobalt-iron-gold eutectic. The liquid phase thus formed increases the densification process. The density of the sintered alloy is about 95% of theoretical. The theoretical density for the foregoing goldiron-cobalt alloy is 8.95 g./ cc.

Substantially similar results are achieved when a stoichiometric equivalent amount of nickel is used in place of iron in the above process. Additionally the iron or nickel can be deleted with similar results.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. A process for producing a cobalt base alloy containing gold comprising:

(a) forming a solution consisting essentially of a predetermined amount of gold and sufiicient aqua regia to dissolve said gold,

(b) forming a mixture consisting essentially of said solution and a cobalt-containing constituent containing suflicient cobalt to achieve the weight ratio of 4 cobalt to gold that is desired in the cobalt base alloy,

(0) drying said mixture to form a powder,

(d) heating said powder in a reducing atmosphere at a temperature of from about 500 C. to about 1000 C. for a period of greater than about 1 hour to form a partially alloyed powder,

(e) compacting said partially alloyed powder into a shaped article and,

(f) sintering said shaped article in a reducing atmosphere at a temperature of from about 1000 C. to about 1250 C. to densify said article to a density of about 93% of theoretical and to complete the formation of the alloy.

2. A process according to claim 1 wherein said reducing atmosphere is hydrogen.

3. A process according to claim 2 wherein the weight ratio of cobalt to gold is from about 1:1 to about 100:1.

4. A process according to claim 3 wherein the cobalt constituent contains nickel.

5. A process according to claim 3 wherein the cobalt constituent contains iron.

6. A process according to claim 5 wherein the weight ratio of cobalt to iron to gold is about 21 :3: 1, respectively.

References Cited UNITED STATES PATENTS 1,423,338 7/ 1922 Laise 75--211 X 2,498,769 2/1950 Ramage 75-211 3,375,109 3/1968 Peters 75-212 FOREIGN PATENTS 827,016 1/ 1960 Great Britain 75--212 LELAND A. SEBASTIAN, Primary Examiner US. Cl. X.R. 75-21, 221 

